This invention relates to a combined photovoltaicthermal collector useful for converting sunlight into electrical and thermal energy.
Present solar cells typically comprise a semiconductor material such as silicon or gallium-arsenide which includes a p-n junction near the light-receiving surface of the cell. When light has an energy which exceeds the band gap energy of the semiconductor material, electrons and corresponding holes are established and electrons move through the semiconductor material. A photon transformed into a mobile negative charge carrier leaves behind a mobile hole and the light photon is said to have formed a free electron hole pair. When the holes and electrons are prevented from recombining and electrons are collected such as in a circuit outside of the semiconductor, they produce electrical current in the circuit. Thus, in solar cells, electrodes are attached to the top and bottom surfaces of the semiconductor to collect the electrons and to attach to the external circuit. Typically, in the case of silicon, the wavelengths of light capable of producing photons having an energy exceeding the band gap energy are those shorter than about 1.1 microns in wavelength. Other semiconductor materials have different band gap energies. Light having a wavelength longer than that value have insufficient energy to form electron-hole pairs in the silicon semiconductor material. Thus, solar cells include optical coating on the top surface which increase absorbence of the light energy within the desired wavelength range, thereby maximizing the formation of electron-hole pairs. In addition, optical coatings are commonly used to reflect light of undesirable wavelengths such as that within the infrared range in order to reduce excessive heating of the cell. Also, since only .about.10% of absorbed radiation results in electric current, the other 90% is converted to heat. Therefore, it is desirable to re-radiate as much of this heat as possible. This is desirable since increased cell temperatures generally result in a decrease in cell efficiency for producing electricity.
At the present time, there is an incentive to provide in residential and industrial applications solar thermal collectors and solar photovoltaic cells. It would be desirable to provide a solar cell construction suitable for converting light energy into both electrical energy and thermal energy. Furthermore, it would be desirable to provide such a cell construction which requires only a single installation process.